The aim of this proposal is to study the developmental regulation of dendritic spine plasticity and the role of experience in modifying synaptic connections in postnatal life. Using an in vivo transcranial two-photon imaging technique, changes of individual dendritic spines will be followed over extended periods of time in different cell types and in diverse cortical regions. We will determine how and to what degree sensory experience and learning modulate dendritic spine plasticity in the developing and adult cortex. Furthermore, we will investigate whether experience-dependent spine plasticity is compromised in mouse models of Fragile X syndrome and Rett syndrome and if so, whether drug treatment can restore experience-dependent spine plasticity in mutant mice. Together, these studies will provide fundamental insights into how experience and genetic factors shape neural circuits at different stages of life and suggest new strategies for the treatment of mental retardation. PUBLIC HEALTH RELEVANCE: The aim of this proposal is to study the developmental regulation of synaptic plasticity and the roles of sensory experience and motor learning in modifying neuronal connectivity in postnatal life. By taking advantage of in vivo two-photon microscopy, we will determine changes in postsynaptic dendritic spines in response to novel sensory stimuli and skill learning in the living mouse cortex. We will also establish an important link between abnormal dendritic spine dynamics and aberrant circuit formation in mouse models of mental retardation. These studies will reveal how sensory inputs and learning shape neuronal connectivity in the cerebral cortex at different stages of an animal's life and suggest novel strategies for the treatment of mental retardation.